A silver halide photographic material requires high sensitivity, fine graininess and low fog as basic capabilities. A means of satisfying the requirements is a metal doping technique. The metal doping technique is conducted to improve the characteristics of silver halide grains and to increase the efficiency of changing a photoelectron into a developable latent image (quantum sensitivity). In more detail, a metal ion or a metal complex containing a ligand is incorporated into silver halide grains according to the metal doping technique. In other words, the grains are doped with the metal ion or the metal complex. According to the metal doping, the characteristics of silver halide grains can be improved to obtain a required capability of an emulsion.
A general and practical surface latent image type silver halide photographic emulsion further requires hard gradation. Particularly, a light-sensitive material for graphic arts should have hard gradation even though the sensitivity is degraded. The hard gradation means that a characteristic curve of an emulsion (abscissa: exposure, ordinate: optical density) has a steep slope at a gradation portion (straight line portion). A steep rise at the toe (an exposure portion having an optical density lower than the density of the gradation portion) of the characteristic curve is also important about the gradation. A conventional silver halide photographic material such as a color paper also requires the hard gradation, namely the steep rise at the toe of the characteristic curve to obtain a clean and clear image.
Research Disclosure 17643, item IA describes metal ions or metal complexes, which can be incorporated into silver halide grains according to the metal doping technique.
At an initial stage of the metal doping technique, metal complexes of platinum, palladium, iridium, rhodium and ruthenium have been frequently used. These water-soluble complexes are described in U.S. Pat. No. 2,448,060. The metal complexes function as an antifogging agent or a stabilizer after silver halide grains are doped with the complexes. Hexa-coordinated cyano-complex of palladium(IV) further has a sensitizing effect. The complexes disclosed in the Patent have a halide compound (such as chloride or bromide compound) as a ligand.
U.S. Pat. No. 3,690,888 discloses a process for preparation of silver halide containing a polyvalent metal ion. The process includes a step of forming silver halide grains in the presence of a protective colloid substantially consisting of an acrylic polymer. The U.S. Patent discloses bismuth, iridium, lead and osmium ions as examples of the polyvalent metal ion. The Patent further discloses complexes of the metal ions. The complexes have a halide compound (such as chloride or bromide compound) as a ligand.
The above-mentioned documents disclose the effects of the metal ions incorporated into silver halide grains.
Japanese Patent Publication No. 48(1973)-35373 discloses potassium hexacyanoferrate(II) and (III) as dopants containing cyano ion. The effect of the disclosed invention does not relate to the nature of the ligand. The effect is limited to a complex having an iron ion.
U.S. Pat. No. 3,790,390 discloses a spectrally sensitized silver halide emulsion containing a cyano complex of iron(II), iron(III) or cobalt(III).
U.S. Pat. No. 4,847,191 discloses silver halide grains, which are formed in the presence of a rhodium(III) complex having 3, 4, 5 or 6 ligands. This document reports that these grains reduce high intensity reciprocity law failure of a silver halide emulsion.
The above-mentioned documents also disclose the effects of the metal ions incorporated into silver halide grains.
Japanese Patent Provisional Publication Nos. 2(1990)-20853 and 2(1990)-20854 disclose silver halide grains, which are formed in the presence of a complex of rhenium, ruthenium, osmium or iridium having 4 or more cyano ligands. As is described in these documents, the emulsion containing these grains has an effect of improving stability about sensitivity. The emulsion has another effect of reducing low intensity reciprocity law failure.
Japanese Patent Provisional Publication Nos. 2(1990)-20852, 2(1990)-20855, 3(1991)-118535 and 3(1991)-118536 disclose a combination of a cyanide ion and a cross-lining ligand (e.g., NO, NS, CO, (O)2).
The above-mentioned documents propose a new concept that seven vacant lattices of one silver ion and six neighboring halide ions are occupied with a hexa-coordinated metal complex consisting of one metal ion and six ligands.
Japanese Patent Provisional Publication No. 6(1994)-51423 (corresponding to European Patent Publication No. 0573066A1) discloses an internal latent image type direct positive silver halide emulsion doped with a hexa-coordinated cyano-metal-complex. The metal is chromium, manganese, cobalt, iridium, ruthenium, rhodium, rhenium or osmium. The effect of the disclosed invention is to obtain an internal latent image type direct positive silver halide of high sensitivity as well as hard gradation.
The internal latent image type silver halide emulsion forms a latent image mainly inside silver halide grains by light exposure. A direct image is formed by the specific character of the internal latent image type emulsion. The image forming process comprises imagewise exposing to light a photographic material having the emulsion and developing the material with a surface developing solution (which selectively does not develop silver halide grains having an internal latent image) while uniformly exposed to light the material or treating the material with a nucleating agent. The internal latent image type direct positive silver halide emulsion has a relatively low sensitivity (compared with the below-mentioned surface latent image type silver halide emulsion) because of the above-mentioned complicated process. Therefore, a high sensitivity is usually required about improvement of the internal latent image type direct positive silver halide emulsion.
On the other hand, a general silver halide emulsion is a surface latent image type, which forms a latent image mainly on the surface of silver halide grains by light exposure. An image forming process using the surface latent image type emulsion comprises imagewise exposing to light a photographic material having the emulsion and developing the material with a developing solution to develop silver halide grains having a surface latent image selectively. The surface latent image type silver halide emulsion has a relatively high sensitivity. Accordingly, photographic characteristics other than sensitivity are sometimes improved, even though the sensitivity is degraded. For example, gradation is hardened for some use of the photographic material while desensitizing the emulsion. The effect of hard gradation with desensitization (desensitizing hard gradation) is different from the effect of the invention disclosed in Japanese Patent Provisional Publication No. 6(1994)-51423, which is not accompanied by desensitization.
As is described above, the internal latent image type emulsion is completely different from the surface latent image type emulsion in the basic image forming mechanism. Therefore, the metal doping effect of the internal latent image type emulsion is sometimes different from the effect of the surface latent image type emulsion.
The metal doping technique is conducted to change the efficiency of changing a photoelectron into a developable latent image. The effect of the metal doping depends on whether the latent image is present inside or surface of the grains.